The Boston Marathon is a high-profile event with a global feel. It is no surprise it would be attractive to a terrorist with an agenda, or no agenda at all. The world was watching, and someone wanted to put on a terrifying show. That President Obama did not say the word “terror” or “terrorism” in his initial statement is of little import. It happened: A name is just a name.

I happened to be just a few miles away when the explosions occurred, preparing to teach a class with the top leadership from the Federal Emergency Management Agency. They had just left the Marathon, but then, of course, we all turned around after hearing the news. Only an hour after the explosions, the scene was calm beyond any expectations. Runners were walking away from the finish line, which was now a crime scene. The police smartly pushed all the spectators out of the area, farther and farther from the focal point, in a controlled evacuation. The medical tent, usually reserved for those who were in pain from running, was quickly being turned into a place to help those who were injured by explosions.

The speed of evacuation, and the intensity of the response, were the products of months of planning for the Marathon. Public-safety officials were on high alert because any event with lots of people around is sure to have safety issues. But no marathon can ever be totally secure. The length of the event is just too long. Thousands of people working safety and security cannot stop one person or a few people from working their way toward the crowded finish line.

There are too many runners, too many spectators, and too much activity to make it truly secure. This is a consequence of what makes a marathon so appealing: It is a spectator event with no doors.

Until definitive information emerges, it’s pointless to speculate on who did or didn’t do this. The Oklahoma City bombing was first blamed on men dressed in “Arab garb.” The thirst for a quick and easy explanation leads everyone astray. Foreign terrorists are much less organized than they used to be, so the apparent lack of sophistication of the incendiary devices doesn’t necessarily point to a domestic attacker. But the explosions happened during a particularly local celebration, suggesting a fair amount of pre-planning, and the devices were targeted at areas with high volumes of people.

The same things that made Boylston Street an attractive terrorist target will make it difficult for the perpetrator or perpetrators to escape unseen. Since the event is practically made for photographs and video, there is little doubt that people who were there may have evidence in their cameras and iPhones that they should review immediately.

Government officials already have two key sources of evidence: Forensic analysis from the bomb, which will show what kind of attack it was, will be combined with eyewitness reports of who was around and what they might have been doing. The finish line is secured for many days before the event, so chances are that the attack took place just as the scene got very hectic.

The investigation will include two streams of intelligence: foreign and domestic. The FBI will have the lead on both, with the support of a web of federal, local, and state agencies. Even with no person or group taking credit (an indicator of domestic, not international, ties), this isn’t an attack that leaves no trace.

The full-court press now on display by local, state, and federal officials is a sign of competence. There is no such thing as overkill at this stage, not in the immediate hours after the event. A show of force coupled with a sense of caution is wise, even as rumors swirl that can’t be confirmed right now.

The specifics will come in time, and the city will learn from them. And, we will welcome back, same time next year, those who so deserved to cross that finish line.

Juliette Kayyem is a former assistant secretary of the U.S. Department of Homeland Security and former security adviser to Gov. Deval Patrick. She is a columnist for The Boston Globe.